Optimal Control of Solar Sludge Dryers

Optimal control (OC) methodology is used to develop a control policy for a batch-operated solar sludge dryer with ventilation as the dominant control. Batch performance criteria and instantaneous optimization criteria are developed for two economic environments: quota limited and capital limited. The general formulation, as well as simplifications resulting from the multiplicative form of the evaporation (drying) rate model, are presented. The multiplicative form results in a single constant number, called here “control intensity,” which is used to guide the on-line control decisions. These decisions turn out to be independent of the dry solids content (DSC) of the sludge. Further simplification, by assuming a strictly constant weather, is used to demonstrate the general effects of the economic and physical (weather) environment on the solution.

The OC approach is used to address, via simulation under realistic weather conditions, several design, operation, and pricing problems. In particular, the following have been considered: sizing of the ventilation fans, determination of a fair fee for sludge disposal, finding the best final DSC, and evaluating the effect of the price of electricity. Regarding the particular location represented by the data, it has been shown that the installed capacity of the ventilation fans should be increased and that there seems to be sufficient economic incentive for solar drying.     

Optimal Control of Continuous Infrared Dryers

In this work, the optimal control policies of radiant temperature versus dryer length are determined for the continuous, steady-state operation of infrared dryers. The optimal control objective is to minimize the product humidity subject to a mathematical model of infrared dryers as well as a set of process inequality constraints. A robust optimal control method based on genetic algorithms is applied. Multiple air injections are also considered. The optimal results show considerable reduction in product humidity. Comparisons with corresponding base cases (using maximum possible uniform radiant temperatures) indicate improvements greater than 50% with the application of optimal control policies.     

Optimal Control of Continuous Infrared Dryers

In this work, the optimal control policies of radiant temperature versus dryer length are determined for the continuous, steady-state operation of infrared dryers. The optimal control objective is to minimize the product humidity subject to a mathematical model of infrared dryers as well as a set of process inequality constraints. A robust optimal control method based on genetic algorithms is applied. Multiple air injections are also considered. The optimal results show considerable reduction in product humidity. Comparisons with corresponding base cases (using maximum possible uniform radiant temperatures) indicate improvements greater than 50% with the application of optimal control policies.     

Modeling Solar Drying Rate of Wastewater Sludge

Efficient solar drying requires that the drying rate is quantitatively known as a function of the environment and the control. To develop a drying-rate model for wastewater sludge, data were collected at a solar drying installation in Füssen, Germany. In this solar dryer, wet sludge is uniformly spread over a concrete floor under a greenhouse-like transparent cover. The sludge is mixed mechanically several times a day by an autonomous robot (electric mole®), the structure is fan-ventilated horizontally, and the indoor air is mixed by electric fans. Data of evaporation rate, environmental conditions, and control operations were collected over three drying cycles. Evaporation rate via sludge sampling and via vapor balance across the structure compared favorably, justifying the use of hourly vapor-balance data. Four types of prediction models were considered: physical, additive, multiplicative, and neural network. The multiplicative model has been selected for potential implementation. The most important predictors of evaporation rate, for the conditions under consideration, were (1) solar radiation, (2) outdoor temperature, (3) ventilation rate, and (4) dry solids content of the sludge. Air mixing is an order of magnitude less effective (per unit of air discharge) than ventilation.     

Sewage Sludge Solar Drying: Experiences from the First Pilot-Scale Application in Greece

Increasingly strict regulations governing sludge management have raised interest in drying technologies. The feasibility of sewage sludge solar drying was experimentally evaluated in a 66-m² pilot-scale greenhouse plant under typical weather conditions in Greece. The greenhouse was equipped with ventilation fans to maximize the drying process efficiency and a turning drum for efficient sludge mixing. The obtained results proved the applicability and the high performance of the solar drying technology. The time necessary to achieve a dry product with a dry solids content up to 95% ranged between 8 and 31 days, depending on the weather conditions. During drying, sludge organic matter was reduced by 5–21%, and total and fecal coliform content was also decreased up to three orders of magnitude. By taking into consideration the sludge content in heavy metals, the final product can partially or totally replace commercially available inorganic fertilizers in agricultural applications, in accordance with the restrictions imposed by national and European regulations. Based on a preliminary cost analysis concerning the construction of a solar drying facility covering a sum of 80,000 population equivalent (PE), a corresponding capital cost of 24 €/PE is anticipated.     

Drying Kinetics and Characteristics of Sewage Sludge/Rice Straw Mixture

This study aimed at investigating thin-layer drying characteristics of sludge mixed with different rice straw contents (1, 2, 3, and 5% wb). The experimental results showed that adding 1, 2, and 3% (wb) rice straw to sludge is helpful to improve the drying rate of sludge as the surface area and cracks increase but the drying rate of the sludge mixed with 5% (wb) rice straw decreases with a reduction in thermal conductivity. When the content of rice straw is 2% (wb), the increased amplitude of drying rate is the highest, 14.6, 14.8, 16.0, and 17.6% at 100, 120, 140, and 160°C, respectively. The model proposed by this study showed better prediction compared with the other models and satisfactorily described the drying characteristics of sludge/rice straw mixture as well and is applicable under the given experimental conditions. The values of R², root mean square error (RMSE), and χ² varied from 0.99935 to 0.99992, 0.00295 to 0.00830, and 0.00001 to 0.00007, respectively.     

Atomization and Drying Characteristics of Sewage Sludge inside a Helmholtz Pulse Combustor

Sewage sludge of pasty consistency was atomized and dried directly using a high-temperature, highly turbulent gas flow in the tailpipe generated by a Helmholtz gas-fired pulse combustor (PC). Parametric studies were carried out to investigate the effect of drying pipe length and sludge feed tube size on the PC sludge drying process. Experimental results showed that the pulsating gas stream can disperse the sludge into small particles with a narrow size distribution between 0.01 and 4 mm and the granular sludge was dried rapidly due to the increased particle surface area and the high temperature of the drying medium. PC drying of sewage sludge was compared to convective- and microwave-drying processes. The application potential of the PC sludge-drying process is discussed based on the experimental results.     

污泥干燥机在污泥干化焚烧工艺中的应用

污泥干化焚烧工艺综合考虑了污泥处置的可靠性、先进性、适用性、安全性与经济性等原则,将是现阶段我国一线城市污泥治理的主流工艺路线。本文通过对不同种类的污泥干燥机进行综合性能比较,推荐了作为污泥干化焚烧工艺的主流干燥机。国产干燥机在经过对国外同类设备的消化吸收后,今后将在国内污泥干化领域占据一席之地。     

城市污泥处理利用现状研究

城市污水处理厂污泥产量巨大且成分复杂,如何把污泥进行合理利用已经越来越受人们的关注。描述了目前城市污水处理厂污泥处理与综合利用的现状。并介绍了几种新兴的污泥处理工艺。为今后污泥的资源化利用提供参考。     

城市污泥快速干燥工艺研究进展

热干化是一种城市污泥安全处置的重要途径。塑化是城市污泥热风干燥常见现象,对污泥干燥过程产生重要负面影响。污泥塑化阻碍了内部水分向外迁移,增加污泥干燥时间和干燥能耗。本文概括了近三年天津科技大学机械工程学院课题组在避免或降低污泥"塑化"影响,发展污泥快速干燥工艺的研究进展。新污泥干化工艺包括城市污泥油炸干燥和脉动燃烧雾化干燥。     

Investigation on Convective Drying of Mixtures of Sewage Sludge and Sawdust in a Fixed Bed

This work is part of a project aiming to develop a renewable fuel for gasification purposes, through convective drying of sludge/wood mixtures. The first step consists of characterizing the behavior of sawdust/sludge mixtures during the application of convective drying. The influence of the mixing step (no mixing versus 30 s at 40 rpm), as well as the sawdust : sludge ratio (1:9, 2:8, 3:7, and 4:6 on a dry basis) and the effect of drying temperature (50, 80, and 110°C) were investigated. In this study, X-ray tomography, a noninvasive imaging technique, is used to assess changes in the volume, void, and exchange surface at the beginning and the end of the drying process. Results first confirm the importance of the mixing step on the drying behavior: the drying rate of the mixed sludge is lower than that of the original sludge. Nevertheless, the addition of sawdust is shown to have a positive impact on the drying process from a mass ratio of 2:8, with drying rates higher than that of the original sludge. With increasing amounts of sawdust, the initial and final bed volumes, initial and final total exchange surfaces, and initial void fraction increase linearly, but the bed volume shrinkage and final void fraction decrease linearly.     

New Objectives of Sludge Treatment Considering a Safe Sludge Removal

Sewage sludge consists of valuable components such as nutrients as well as harmful substances, e.g. heavy metals. For sludge with a low concentration of pollutants, its utilization in agriculture is the best way of sludge reuse. Carbon recycling is also possible by using disintegration in combination with anaerobic degradation processes. The other important compound in sludge is phosphorus, which is a limited resource on earth. Recycling options for phosphorus are also described in the paper. A safe disposal of the residues can be achieved by using incineration.     

Optimal Control of the Secondary Drying Stage of Freeze Drying of Solutions in Vials Using Variational Calculus

Abstract:

The problem of operating freeze drying of pharmaceutical products in vials loaded on trays of freeze dryer to obtain a desired final bound water content in minimum time is formulated as an optimal control problem. Two different types of freeze dryer designs were considered. In the type I freeze dryer design, upper and lower plate temperatures were controlled together, while in the type II freeze dryer design, upper and lower plate temperatures were controlled independently. The heat input to the material being dried and the drying chamber pressure were considered as control variables. Only the scorch temperature was considered as a constraint on the system state variables during the secondary drying stage, because all the free water content (frozen water) is removed from the solid matrix during the primary drying stage of freeze drying. Necessary conditions of optimality for the secondary drying stage of freeze drying process in vials were derived and presented by using rigorous multidimensional unsteady-state mathematical models. The theoretical approach presented in this work was applied in the freeze drying of skim milk. Significant reductions in drying times of the secondary drying stage of the freeze drying process in vials were observed and more uniform bound water and temperature distributions in the material being dried were obtained compared to the conventional operational policies.     

Sewage Sludge Dewatering Process Using Liquefied Dimethyl Ether as Solid Fuel

This article describes the dewatering of sewage sludge using liquefied dimethyl ether (DME), which clarified the effect of sludge type and its particle size. Changes in the heating value of the dewatered sludge were also examined. In addition, the change in dewaterability upon reuse of the liquefied DME was investigated experimentally.

Regardless of the nature of the sludge, its dewatering behavior was similar. The amount of dewatered water could be increased by decreasing the diameter of the sludge cake. Complete drying of the sludge cake via DME dewatering resulted in retention of approximately 96% of the heating value (17.7 MJ/kg-sludge) of the original sludge cake dry base (18.4 MJ/kg db). The DME could be recovered after extraction, and reuse experiments showed that liquefied DME dewatering performance was unchanged after five reuses.     

城镇污泥干燥过程中胶粘性的研究

主要对污泥的干燥性质、污泥在干燥过程中所发生的胶粘性的变化作了机理性的研究,在此基础了提出了一些对污泥干燥过程中胶粘性的特点,并对如何解决和避免胶粘性提出了建议;针对污泥这种含水量大、粘性大的特殊物料的干燥,选用普通回转圆筒进行干燥,对干燥后物料、干燥过程粘性的变化等作了分析,并在此基础上了对设备进行了改造,使之能够解决和避免在回转圆筒干燥机中干燥污泥产生的胶粘性问题,主要的改造是在回转圆筒的进料端加上了自清理装置,以及横贯圆筒中心的打散装置,以此产生了新的回转圆筒设备——带自清理的打散回转圆筒干燥机,并且作了工业模拟试验。     

Experimental Evaluation of Solar Kiln for Drying Wood

Semi-greenhouse solar dryer was designed and constructed in the Solar Energy Department, National Research Center, Cairo, Egypt. It consists of steel frame structure and all the sides were insulated and covered by galvanized iron sheet. The roof faces south with a tilt angle of 30° and covered by glass sheets. Corrugated iron sheets, painted black on the top, were fixed beneath the glass roof as an absorber. The humidity inside the solar kiln is controlled by inlet vent at south wall and two outlet vents at north wall. Data Capture System was designed to ease the measurements and to capture the measured data automatically and continuously for 24 h a day. The selected local wood type is Casuarina, which is common in Egypt, and it is commonly used in many simple industries. The experimental results with this type of wood show considerable agreements with the simulated ones. A comparison was made between experimental results of solar wood drying and traditional air-drying. The results show that timber boards were dried inside solar kiln to moisture content of 12% within 17 days, where air dying moisture content was limited to 20% for the same period.     

Control of Spouted Bed Dryers

During the last 8 years, the Drying Center at Universidade Federal de São Carlos has been investigating the control problems of drying process of pasty materials associated with the spouted bed dryer. Starting with structures involving PI controller, we also implemented advanced MPC control techniques and real time optimization (this last one based on simulations only). Different control structures were implemented in a pilot spouted bed dryer where the behavior of the system was tested for water and homogenized chicken eggs. Based on promising results, the perspective is to use an adequate control system in a spouted bed scale‐up.     

Optimal Control of the Primary Drying Stage of Freeze Drying of Solutions in Vials Using Variational Calculus

The problem of operating freeze drying of pharmaceutical products in vials placed in trays of a freeze dryer to remove free water (in frozen state) at a minimum time was formulated as an optimal control problem. Two different types of freeze dryer designs were considered. In type I freeze dryer design, upper and lower plate temperatures were controlled together, while in type II freeze dryer design, upper and lower plate temperatures were controlled independently. The heat input to the material being dried and the drying chamber pressure were considered as control variables. Constraints were placed on the system state variables by the melting and scorch temperatures during primary drying stage. Necessary conditions of optimality for the primary drying stage of freeze drying process in vials are derived and presented. Furthermore, an approach for constructing the optimal control policies that would minimize the drying time for the primary drying stage was given. In order to analyze optimal control policy for the primary drying stage of the freeze-drying process in vials, a rigorous multi-dimensional unsteady state mathematical model was used. The theoretical approach presented in this work was applied in the freeze drying of skim milk. Significant reductions in the drying times of primary drying stage of freeze drying process in vials were obtained, as compared to the drying times obtained from conventional operational policies.     

Optimal Control of the Primary Drying Stage of Freeze Drying of Solutions in Vials Using Variational Calculus

Abstract

The problem of operating freeze drying of pharmaceutical products in vials placed in trays of a freeze dryer to remove free water (in frozen state) at a minimum time was formulated as an optimal control problem. Two different types of freeze dryer designs were considered. In type I freeze dryer design, upper and lower plate temperatures were controlled together, while in type II freeze dryer design, upper and lower plate temperatures were controlled independently. The heat input to the material being dried and the drying chamber pressure were considered as control variables. Constraints were placed on the system state variables by the melting and scorch temperatures during primary drying stage. Necessary conditions of optimality for the primary drying stage of freeze drying process in vials are derived and presented. Furthermore, an approach for constructing the optimal control policies that would minimize the drying time for the primary drying stage was given. In order to analyze optimal control policy for the primary drying stage of the freeze-drying process in vials, a rigorous multi-dimensional unsteady state mathematical model was used. The theoretical approach presented in this work was applied in the freeze drying of skim milk. Significant reductions in the drying times of primary drying stage of freeze drying process in vials were obtained, as compared to the drying times obtained from conventional operational policies.     

Dimensionless Formulation of Convective Heat Transfer in Fry‐Drying of Sewage Sludge

Fry‐drying is an alternative for heat and mass transfer intensification. The process reuses waste oil as a heating medium for drying by contact with the wet sludge. At the end of the process, a stable derived fuel is obtained, a granular solid composed of the dried indigenous sewage solid and the impregnated oil. The fry‐dried sludge is storable and transportable without any pathogen elements. Knowledge about heat and mass transfer rates during the frying process is essential in order to assess the quality of the final product such as calorific value, oil uptake, porosity changes, etc. The heat transfer properties including transfer by free convection between the solid and the frying oil are fundamental for the process design and manufacturing of the fry‐dried product. The convective heat coefficient by temperature measurement and overall energy balance calculation is determined. The heat flux is calculated from the fry‐drying kinetics including moisture loss and oil intake kinetics. Various hydrodynamic regimes for convective heat transfer during the frying process are discussed (non‐boiling, boiling, and low‐boiling regime). A dimensionless formulation for estimating the convective transfer is proposed.